EXCHANGE 


A  Study  of  the  Vapor  Pressures  of  Cer- 
tain Hydrated  Metallic  Sulphates 


, 


DISSERTATION 


SUBMITTED  IN  PARTIAL  FULFILLMENT  OF  THE  REQUIREMENTS 

FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY   IN  THE 

FACULTY  OF  PURE  SCIENCE,  COLUMBIA  UNIVERSITY 


BY 


Eric  Randolph  Jette,  B.S.,  M.A. 


NEW  YORK  CITY 
1922 


A  Study  of  the  Vapor  Pressures  of  Cer- 
tain Hydrated  Metallic  Sulphates 


DISSERTATION 

SUBMITTED  IN  PARTIAL  FULFILLMENT  OF  THE  REQUIREMENTS 

FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY   IN  THE 

FACULTY  OF  PURE  SCIENCE,  COLUMBIA  UNIVERSITY 


BY 
Eric  Randolph  Jette,  B.S.,  M.A. 


NEW  YORK  CITY 
1922 


Dedicated  to  my  parents 


ACKNOWLEDGMENT 

It  is  with  pleasure  that  the  author  acknowledges  his  indebtedness  to 
Professor  C.  D.  Carpenter  for  his  constructive  criticism  and  generous 
cooperation  during  the  progress  of  this  investigation. 

CXCHANOB 


CONTENTS 

SUBJECT  MATTER 

Page 

Abstract  of  the  Dissertation 6 

Introduction 7 

The  Static  Method  and  Difficulties  in  its  Use 8 

The  Use  of  the  Static  Method  by  Various  Investigators 11 

Description  of  Apparatus 12 

Experimental  Procedure 15 

Discussion  of  the  Accuracy  of  the  Results 18 

Interpretation  of  the  Results 20 

Application  of  the  Results , 21 

Summary 25 

Vita 27 

ILLUSTRATIONS 

Fig.  0.     Typical  Static  Instruments 9 

Fig.  1.     Thermostat  for  Higher  Temperature 13 

Fig.  2.     Tensimeter  Used  in  the  Investigation 14 

Fig.  3.     Rate  of  Approach  to  Equilibrium 16 

Fig.  4.     Curves  for  Vapor  Pressure  Data 18 

Fig.  5.     Solubility  Curves  for  MgSO4  and  CdSO4 20 

Fig.  6.     Log  p —  lines 24 


531857 


ABSTRACT  OF  DISSERTATION 

1.  What  was  attempted. 

An  attempt  -was  made  to  measure  the  vapor  pressure  of  certain 
hydrated  salts  at  several  points  over  a  considerable  range  of  tempera- 
ture in  order  to  study  the  phenomenon  as  a  function  of  the  tempera- 
ture. 

2.  What  were  the  methods? 

(a)     Special  emphasis  was  placed  upon  the  construction  of  a  thermo- 
stat to  operate  at  the  higher  temperatures. 

(6)     Improvements  in  the  technique  in  the  use  of  the  tensimeter  were 

employed. 
'     (c)     Equilibrium  was  approached  from  higher  and  lower  pressures. 

3.  In  how  far  were  the  attempts  successful? 

Considerable  data  has  been  obtained  on  six  hydrates,  and  on  the 
saturated  solution  of  some  of  them  above  the  transition  point.  The 
experimental  errors  usually  encountered  in  determining  vapor  pressures 
have  been  practically  eliminated. 

4.  What  contributions  actually  new  to  the  science  of  chemistry  have  been 

made? 

(a)  The  collection  of  complete  vapor  pressure  data  for  several  hy- 
drates and  saturated  solutions  has  been  accomplished. 

(b)  Certain  new  transition  points  have  been  located  by  this  method : 

CoS04(7H20-6H20)  at  45.1°  C 

CdS04(-|-H20-lH20)  at  41.5°  C. 

•H.       ..,-.',  o 

>'.:  The  transition  point  of 

MgSO4(7H2O-6H2O)  at  48.4°  C 

previously  found  by  Van  der  Heide  between  48°  and  48.5°  has 
been  verified. 

(c)  It  has  been  shown  that  the  vapor  pressures  of  the  hydrates  and 
saturated  solutions  investigated  may  be  expressed  as  a  continuous 

1 
function  of  log  p  and  — ,  in  the  same  manner  as  the  vapor  pressure 

of  water  and  of  solutions  of  various  concentrations. 

(d)  The  results  have  been  calculated  for  the  Heat  of  Vaporization. 


A  STUDY  OF  THE  VAPOR  PRESSURES  OF  CERTAIN  HYDRATED 
METALLIC  SULPHATES 

Introduction 

That  many  substances  are  volatile  or  decompose  to  give  one  or  more 
gases,  and  that  the  tendency  to  do  so  is  a  definite  function  of  the  nature 
of  the  substance  and  the  conditions  imposed  upon  them,  and  that  this 
tendency  is  measurable  in  terms  of  the  pressure  of  the  gas  phase  (usually 
called  the  vapor  pressure  -of  the  system)  is  well  known.  The  tendency 
for  certain  salt  hydrates  to  gain  or  lose  water  is  cited  in  the  early  literature 
of  chemistry.  Vogel1  in  1818  reports  that  blue  vitriol  effloresces  rapidly 
over  sulphuric  acid  or  fused  calcium  chloride,  and  little  or  not  at  all  in  the 
air.  In  1838  Watson2  made  what  seems,  to  be  among  the  first  attempts 
to  measure  quantitatively  the  tendency  for  a  salt  hydrate  (sodium  car- 
bonate) to  effloresce,  and  expressed  his  observations  in  terms  of  the  humidity 
of  the  air  in  which  efflorescence  begins.  Mitscherlich3  describes  the 
measurement  of  the  lowering  of  the  mercury  column  in  a  "Torecelli  Bar- 
ometer" when  a  crystal  of  sodium  sulphate  is  introduced  into  the  free  space 
above  the  mercury. 

Wilson4  has  recently  pointed  out  that  the  work  on  the  vapor  pressure 
of  salt  hydrates  "prior  to  1875  had  little  scientific  value  owing  chiefly  to 
the  hazy  notions  which  prevailed  as  to  the  nature  of  the  phenomenon." 
Although  Gibbs5  formulated  the  phase  rule  in  1877,  it  was  nearly  fifteen 
years  later  before  its  merits  were  recognized.  It  seems,  however,  that  dur- 
ing this  period  several  investigators  of  the  vapor  pressure  of  salt  hydrates 
recognized  the  necessity  of  the  presence  of  two  solid  phases  in  order  to  de- 
fine the  point  of  equilibrium,  and  as  shown  by  the  nature  of  their  results, 
real  progress  may  be  said  to  date  from  this  period.  An  understanding  of 
the  phenomenon  resulted  in  the  development  of  new  methods  for  measuring 
the  vapor  pressure  at  the  point  of  equilibrium. 

Although  numerous  investigators  have  studied  the  vapor  pressure  of 
salt  hydrates  as  a  function  of  the  temperature,  the  data  is  still  very  in- 
complete. Much  of  the  work  has  been  done  in  order  to  test  the  method 
proposed,  or  as  Wilson6  points  out,  "for  the  express  purpose  of  substan- 
tiating van't  Hoff's  equation  and  other  thermodynamic  laws  for  the  case 
of  salt  hydrates,"  rather  than  to  collect  extensive  data  on  a  single  hydrate. 
It  was  therefore  only  necessary  to  obtain  a  result  at  one  or  two  tempera- 
tures. The  temperatures  usually  chosen  were  25°  C  or  the  boiling-point 

1  Schweigger's  Journal,  22,  160  (1818). 

2  J.    Prakt.    Chem.,    14,    112    (1838). 

3  Lehrbuch  Der  Chemie,  4th  Edition. 

4  /.  Am.  Chem.  Soc.,  42,  704  (1921). 

5  Trans.  Connecticut  Acad.,  1874-1878. 

6  Loc.  cil. 


of  some  liquid,  and  in  only  a  few  cases  were  experiments  carried  out  at 
other  temperatures.  Wiedemann7  carried  some  measurements  up  to 
98.5  °  C,  Schottky 8  a  few  at  90  °  C,  and  Derby  and  Yngve9  from  10  °  to  140  °  C. 

Many  methods  for  determining  the  vapor  pressure  of  substances  have 
been  developed.  Extensive  experiments,  comprehensive  discussions  and 
bibliographies  of  the  more  important  investigations  have  been  given  by 
Johnson10  on  hydroxides  and  carbonates,  by  Smith  and  Menzies11  on  gen- 
eral methods  as  applied  to  all  phases  of  the  subject,  by  Menzies12  on  "Ap- 
parent Anomalies  Outstanding  in  the  Results  of  Measurements  of  Dis- 
sociation Pressures,"  and  by  Wilson13  in  a  recent  paper  on  "Some  New 
Methods  for  the  Determination  of  the  Vapor  Pressure  of  Salt  Hydrates." 
It  seems  agreed  that  the  methods  can  best  be  classed  under  three  heads. 
(1)  Static,  (2)  Dynamic,  and  (3)  Indirect.  We  shall  not  attempt  a  general 
discussion  of  these  methods  as  the  discussions  just  cited  seem  adequate. 

A  careful  consideration  of  the  literature,  which  revealed  the  lack  of  ex- 
perimental data  on  the  vapor  pressure  of  salt  hydrates  over  a  wide  range  of 
temperatures;  the  disagreement  in  the  results  of  previous  investigators; 
in  most  cases,  failure  to  study  the  approach  of  equilibrium  from  both  sides ; 
the  usual  assumption  that  equilibrium  is  reached  quickly ;  and  many  other 
conspicuous  cases  of  oversight  or  error,  made  it  seem  worth  while  to  make 
a  careful  experimental  study  of  the  vapor  pressure  of  a  few  of  these  salt 
hydrates.  Preliminary  experiments  with  certain  hydrated  sulphates 
demonstrated  that  the  equilibrium  is  reached  slowly,  especially  at  the  lower 
temperatures  at  which  most  previous  investigators  worked.  It  was  there- 
fore decided  that  some  form  of  the  static  method  would  best  serve  our 
purpose,  as  when  once  set  up,  it  might  be  used  indefinitely.  In  order  that  our 
method  and  apparatus  may  be  better  understood,  we  shall  briefly  discuss 
"The  Static  Method  where  a  Confining  Liquid  is  Employed"  and  the 
"Difficulties  Encountered,"  and  "The  Use  of  the  Static  Method  by  Previous 
Investigators." 

The  Static  Method  Where  a  Confining  Liquid  is  Employed  and  the 

Difficulties  Encountered  in  its  Use 

Types  of  Apparatus. — An  idea  of  the  general  nature  of  the  different 
types  of  apparatus  designed  for  vapor  pressure  measurements,  by  means 
of  a  confining  liquid,  may  be  most  easily  understood  by  noting  the  sketches 
in  Fig.  0.  In  (a)  the  substance  is  introduced  into  the  free  space  above  the 

7  Wied.  Ann.,  17,  561  (1882). 

8  Zeit.  physik.  Chem.,  64,  415  (1908). 

9  /.  Am.  Chem.  Soc.,  38,  1439  (1916). 

10  Zeit.  physik.  Chem.,  62,  330  (1908). 

11  /.  Am.  Chem.  Soc.,  32,  907;   32,  1412;   32,  1434;   32,  1449,  and  32,  1541  (1910). 

12  /.  Am.  Chem.  Soc.,  42,  1915  (1920). 
11  Loc.  cit. 


9 


mercury  column  in  a  Torecelli  Barometer  and  the  vapor  pressure  is  read 
directly  by  the  lowering  of  the  column  of  mercury.  In  (6)  the  substance 
is  introduced  into  the  shorter  arm  of  a  J  tube,  and  the  confining  liquid 
into  the  U.  Both  arms  are  then  evacuated  and  sealed  off.  The  pressure 
is  then  read  directly  in  terms  of  the  difference  in  the  height  of  the  liquid 
columns  in  the  U.  In  (c)  the  substance  is  introduced  into  the  bulb  on  the 
left  and  after  adding  mercury  as  confining  liquid,  the  bulb  is  evacuated  and 
sealed  off.  The  pressure  is  determined  by  noting  the  height  of  the  columns 
and  adding  or  subtracting  as  the  case  may  require  the  difference  from  the 
barometer  reading.  In  (d)  the  substance  is  placed  in  the  bulb  on  the  left 


Fig.  0. 

and  the  confining  liquid  into  the  U.  The  open  end  of  the  instrument  is 
then  attached  to  a  vacuum  pump,  tilted  on  one  side  and  evacuated.  It 
is  then  brought  to  an  upright  position  so  the  free  spaces  in  the  two  arms  are 
separated  by  the  confining  liquid.  In  order  to  make  readings,  the  open 
end  must  be  connected  to  a  manometer,  and  vacuum  and  pressure  pumps 
so  that  the  pressure  in  the  open  arm  can  be  made  equal  to  that  of  the  vapor 
pressure  of  the  substance.  When  the  columns  are  at  the  same  height,  the 
manometer  gives  the  pressure  exerted  by  the  gas  liberated  from  the  sub- 
stance. In  (d)  the  substance  is  placed  in  one  bulb,  the  confining  liquid 
into  the  U,  and  a  reference  substance,  the  vapor  pressure  of  which  is  known, 
in  the  other.  Both  bulbs  are  simultaneously  evacuated  and  sealed  off 
while  keeping  the  instrument  in  an  upright  position.  The  pressure  is  then 
determined  by  adding  to  or  subtracting  from  the  difference  in  the  height  of 


10 

the  columns  of  the  confining  liquid,  the  known  pressure  of  the  ref- 
erence substances,  depending  on  whether  its  vapor  pressure  is  greater 
or  less  than  that  of  the  reference  substance.  Many  modifications  of  the 
types  illustrated  have  been  used,  but  those  described  above  embody  the 
general  principles  employed  in  all.  Those  who  have  used  any  one  of  these 
types,  selected  it  because  it  could  be  used  to  better  advantage  in  the 
problems  under  consideration.  The  difficulties  which  are  encountered 
in  these  types  and  the  various  modifications  are  about  the  same  and  some 
of  the  most  conspicuous  are  considered  below. 

Enclosed  Gas. — One  of  the  chief  errors  in  the  static  method  may  result 
from  gas  which  is  not  entirely  eliminated  when  the  instrument  and  its 
charge  is  prepared  for  use.  Among  the  most  conspicuous  sources  of  gas 
in  such  an  instrument  are  (a)  upon  the  walls  of  instrument,  (6)  upon  the 
surface  of  the  material  in  the  instrument,  absorbed  or  adsorbed,  (c)  oc- 
cluded mechanically  in  pockets  in  the  substance,  and  (d)  dissolved  in  the 
crystal  or  inclosing  liquid.  In  case  a  differential  method  is  used,  the  same 
troubles  with  gas  may  arise  on  the  side  in  which  the  reference  substance  is 
contained. 

Manometer  Liquid. — The  nature  of  the  manometer  liquid  may  be  such 
as  to  introduce  considerable  error.  An  ideal  manometer  liquid  would  be 
one  that  is  light  in  order  that  small  changes  in  the  pressure  can  be  easily 
recognized;  has  no  vapor  pressure  so  that  the  gas  pressure  represents 
only  that  of  the  substance  under  investigation  as  well  as  to  prevent  its 
distillation  and  subsequent  contamination  of  the  substance ;  is  non-solvent 
for  the  gases  in  order  to  prevent  transmission  of  gases  from  one  side  of  the 
instrument  to  the  other;  and  one  which  does  not  "wet"  the  instrument  as 
snch  a  meniscus  is  easier  to  read  when  the  height  of  the  liquid  column  is 
being  noted. 

Obtaining  Equilibrium. — The  difficulties  in  obtaining  equilibrium 
have  not  been  as  seriously  considered  by  some  investigators  as  they  should 
have  been.  One  of  the  difficulties,  that  of  maintaining  a  constant  tem- 
perature, has  been  extensively  studied  and  met  by  the  design  of  very  effi- 
cient thermostats  adjustable  to  any  temperature  up  to  about  50°  C.  A 
second  difficulty  arises  in  the  choice  of  the  static  method.  If  equilibrium 
is  reached  immediately,  an  isotenoscope  of  the  Smith  and  Menzies14  type 
may  be  used,  but  in  case  the  equilibrium  is  reached  slowly,  it  could  only  be 
used  by  inclosing  the  manometer  in  the  thermostat,  i.  e.,  if  readings  in 
fractions  of  mm.  of  mercury  were  desired.  As  shown  in  Fig.  0  (d)  the 
isotenoscope  is  introduced  into  the  thermostat,  and  its  open  end  is  attached 
through  tubing  to  a  vacuum  bottle,  pump,  and  manometer,  so  that  the  con- 
fining liquid  may  be  leveled  at  (a)  making  the  pressure  equal  on  both 
sides.  Any  change  in  the  room  temperature  would  be  registered  by  a 
14  /.  Am.  Chem.  Soc.,  32,  1413,  Fig.  3  (1910). 


11 

change  of  level  in  the  confining  liquid.  Since  a  manometer  standing  in  a 
room  undergoes  the  same  temperature  change  as  the  room,  a  variation 
of  1  °  C  in  a  room  at  20°  C  changes  the  pressure-2.5  mm.  on  a  system  regis- 
tering 760  mm.  This  would  result  in  a  corresponding  change  in  height  of 
the  columns  of  the  confining  liquid.  In  case  a  light  liquid  were  used,  a 
much  greater  difference  in  the  height  of  the  leveling  liquid  would  result. 
If  the  equilibrium  is  reached  slowly,  a  change  in  the  temperature  of  the 
room  would  therefore  impose  an  excessive  pressure  at  one  time  and  a 
diminished  pressure  at  another.  In  fact,  as  true  equilibrium  is  approached, 
the  reaction  would  be  forced  backward  at  one  time  and  forward  at  an- 
other, and  any  reading  taken  could  be  correct  only  accidentally.  It  is 
therefore  evident  that  a  method  for  use  in  case  of  a  slow  approach  to 
equilibrium  must  be  one  in  which  no  manipulation  will  effect  a  change  in 
conditions  during  the  progress  of  the  reaction. 

The  Use  of  the  Static  Method  by  Various  Investigators 
The  Bremer-Frowein  Tensimeter. — Frowein15  in  studying  the  vapor 
pressure  of  certain  hydra  ted  salts  used  a  differential  method,  illustrated 
in  Fig.  0  (e)  similar  to  one  previously  used  by  Bremer.  This  instrument 
is  known  as  a  Bremer-Frowein  Tensimeter.  Frowein  used  olive  oil  as 
enclosing  liquid  with  concentrated  sulphuric  acid  on  one  side  to  produce  a 
zero  pressure  of  water  vapor  on  that  side..  His  results  have  been  much 
quoted  and  in  spite  of  certain  difficulties  not  overcome  by  him,  the  work  is 
of  considerable  value.  Andreae16  used  the  tensimeter,  with  oil  as  the  con- 
fining liquid,  to  study  the  equilibrium  of  a  hydrate  when  containing  differ- 
ent amounts  of  the  same  two  solid  phases,  by  balancing  one  against  another. 
As  a  result  of  his  experiments,  he  discovered  that  there  were  limits  between 
which  the  compositions  of  the  solid  phase  might  vary,  but  over  which  no 
difference  in  vapor  pressure  could  be  recognized.  Although  the  phase 
rule  was  not  known  to  him,  the  conclusions  given  by  him  coincide  with 
those  required  by  its  application.  Schottky17  used  the  tensimeter  with 
paraffin  oil  as  confining  liquid  to  study  the  approach  of  equilibrium  when 
reached  from  both  sides,  by  putting  the  same  hydrate  in  both  bulbs  and 
heating  one  to  a  higher  temperature  than  that  at  which  the  approach 
to  equilibrium  was  to  be  studied,  but  was  unable  to  satisfactorily  account 
for  some  of  the  difficulties.  Cohen18  used  the  tensimeter  with  mercury 
as  confining  liquid  when  studying  vapor  pressures  of  hydrates,  and  ap- 
plied his  data  to  a  calculation  of  the  heat  of  hydration.  Menzies19  in- 
vestigated the  so-called  "anomalies"  of  other  investigators,  in  some  cases 

™  Zeit.  physik.  Chem.,  I,  5  (1887). 

16  Zeit.  physik.  Chem.,  7,  241  (1891). 

17  Loc.  cit. 

18  Zeit.  physik.  Chem.,  36,  517  (1901). 

19  Loc.  cit. 


12 

repeating  their  work  and  concludes  that  "the  real  facts  exhibit  no  anom- 
alies." This  work  included  the  use  of  the  tensimeter  in  testing  some  of 
Frowein's  results  and  gives  reasons  "for  accepting  the  tensimetric  results 
of  Frowein,  often  regarded  as  standard,  only  with  caution,"  and  a  possible 
explanation  of  some  of  Schottky's  difficulties. 

Open  End  Tensimeter  Attached  to  a  Manometer. — In  measuring  the 
vapor  pressure  of  hydroxides  and  carbonates,  Johnston20  used  a  modi- 
fication of  the  tensimeter,  one  arm  of  which  was  attached  to  a  manometer 
standing  in  a  room,  and  made  his  readings  in  mm.  of  mercury.  He  elim- 
inated much  of  the  error  in  his  results  for  the  hydroxides  by  plotting  the 
observed  temperature  against  the  temperature  of  water  at  which  it  would 
have  the  observed  pressure  for  the  hydroxide  in  order  to  derive  a  formula 
with  which  he  calculated  the  temperature  at  which  the  hydroxide  should 
give  the  observed  pressure.  As  the  corrected  temperatures  varied,  in 
some  cases,  9°  C  from  that  of  the  observed,  the  errors  due  to  change  in 
temperature  of  the  room  and  its  effect  upon  the  equilibrium  as  pointed  out 
in  our  discussion  above  may  be  neglected.  Derby  and  Ingve21  used  the 
isotenoscope  method  described  by  Smith  and  Menzies22  for  measuring  the 
vapor  pressures  of  certain  salt  hydrates.  They  emphasized  that  tempera- 
tures were  accurately  obtained  and  that  they  used  a  paraffin  oil  with  neg- 
ligible vapor  pressure  so  that  the  record  of  change  of  pressure  could  be 
more  readily  recognized,  but  do  not  discuss  the  difficulty  arising  from  the 
change  of  temperature  in  the  room  as  we  pointed  out  in  the  discussion  above. 
It  must  be  assumed  that  the  salts  used  by  them  reached  equilibrium  be- 
fore any  change  in  the  room  temperature  took  place  as  they  give  results 
in  fractions  of  mm.  of  mercury. 

Only  a  few  cases  have  been  cited,  but  sufficient  to  give  a  general  idea  of 
the  nature  of  the  work  already  done  along  the  lines  which  have  been  in- 
volved in  this  investigation.  As  already  pointed  out  the  articles  cited 
have  g9ne  much  more  completely  into  certain  other  phases  of  the  work 
done  by  various  investigators. 

Description  of  Apparatus 

As  this  investigation  has  been  carried  out  in  order  to  obtain  vapor 
pressures  of  certain  hydrated  sulphates  at  several  temperatures  by  allowing 
equilibrium  to  be  reached  from  both  sides,  thermostats  which  gave  constant 
temperature  control  over  long  periods  of  time  were  required.  The  instru- 
ment by  which  the  actual  measurements  were  made  was  designed  so  that 
when  once  set  up,  it  could  be  used  at  various  temperatures  and  over  a  long 
period  of  time. 

20  Loc.  cit. 

21  Loc.  cit. 

22  Loc.  cit. 


13 

The  Thermostats. — Four  thermostats  have  been  used.  A  Freas 
thermostat  Type  5132  (E  and  A  catalogue)  was  used  at  25°  C  and  one  of 
Type  5132/1  from  30°  to  50°  C.  For  temperatures  from  50°  to  65°  C 
a  Pyrex  jar  9^"  X  15"  was  insulated  with  "Magnesia"  \y?  thick  and  regu- 
lated by  means  of  a  Freas  regulator.  A  narrow  window  was  made  in  the 
insulation  so  that  readings  could  be  made.  For  temperatures  from  65° 


Fig.  1. 

up  and  during  the  later  experiments  down  to  60°,  a  special  thermostat 
was  assembled,  Fig.  1.  A  Pyrex  jar  (a),  referred  to  above,  insulated 
with  magnesia  (b)  was  surrounded  by  an  asbestos  box  15"  X  15"  X  15" 
(c)  in  which  an  electric  bulb  (/)  of  appropriate  capacity  could  be  suspended 
to  keep  the  temperature  very  near  that  at  which  the  thermostat  was  set. 


14 


This  box  had  a  small  removable  door  (k)  in  front  of  the  window  (i)  in  the 
magnesia  insulation,  which  could  be  opened  when  readings  were  to  be  taken. 
Water  was  put  into  the  jar  and  covered  with  a  J^"  layer  of  paraffin.  The 
whole  thermostat,  stirrer  and  other  attachments  were  inclosed  in  a  space 
surrounded  and  covered  by  a  large  wool  fire  blanket  supported  by  four 
tall  posts.  This  acted  as  a  second  chamber  in  which  electric  bulbs  could 
be  suspended  to  keep  a  constant  temperature.  The  thermostat  was  thus 
kept  in  a  double  "constant  temperature  room." 

The  regulation  of  the  temperature  was  accomplished  by  a  thermo- 
regulator,  (d),  mercury  in  glass,  making  and  breaking  an  electric  circuit 
by  contact  of  the  mercury  in  a  capillary  with  a  platinum  point.  In  order 
that  the  regulator  should  register  small  temperature  changes  in  all  parts 
of  the  bath,  it  was  constructed  of  Vs"  Pyrex  tubing  having  eight  fingers 
as  shown  in  Fig.  1  (d).  The  electric  circuit  was  attached  to  a  controlling 
device,  designed  by  D.  and  J.  Beaver,23  which  opened  or  closed  the  inter- 
mittent electric  heaters.  These  heaters  (e)  were  made  by  sealing  a  coil 
of  Driver  Harris  wire  in  an  evacuated  Pyrex  glass  tube  15"  long  by  5/s" 
diameter.  One  was  used  as  a  continuous  heater  and  one  or  two,  as  needed, 
as  intermittent. 

The  stirring  was  accomplished  by  a 
turbine  propeller  in  a  tube  14"  X 
11/2I/  and  mounted  as  described  by  Car- 
penter.24 This  produced  a  rapid  circula- 
tion of  the  water  in  the  bath. 

The  Tensimeter. — Fig.  2,  A,  illus- 
trates the  principle  features  of  the  ten- 
simeter.  It  was  of  Pyrex  tubing,  12" 
over  all,  and  except  for  the  bulbs  of 
heavy  wall  4  mm.  I.  D.  tubing.  The 
tensimeter  was  mounted  upon  a  support 
carrying  a  scale  etched  upon  a  strip  of 
milk  glass.  This  strip  was  270  mm.  long 
and  25  mm.  wide  and  calibrated  from  0, 
at  10  mm.  from  the  lower  end,  to  250 
mm.  near  the  upper.  The  mounting  is 
shown  in  Fig.  2  B. 

The  Thermometer. — The  thermom- 
eters  were   all   mercury   in   glass    and 
were  calibrated  every  five  degrees   by 
a  platinum  resistance  thermometer  standardized  as  described  by  the  Bureau 
of  Standards.25 

23  Description  to  be  published. 

2<  Chem.  and  Met.  Eng.,  24,  569  (1921). 

26  Reprint  124;    J.  Am.  Chem.  Soc.,  41,  748  (1919). 


B 


Fig.  2. 


15 

Experimental  Procedure 

Preparation  of  the  Hydrates. — Chemically  pure  hydrates  were  dis- 
solved in  pure  water  and  recrystallized  five  or  six  times.  After  each  crys- 
tallization the  solid  was  drained  from  the  mother  liquor  and  washed.  The 
final  crystallization  was  always  carried  out  by  evaporation  of  the  water 
at  low  temperatures.  The  lower  hydrate  was  obtained  by  gently  heating 
the  higher  until  sufficient  water  had  been  driven  off.  The  water  compo- 
sition of  the  hydrates  used  was  determined  in  all  cases  by  analysis. 

Preparation  of  the  Tensimeter. — The  tensimeter  was  thoroughly 
washed  with  an  alkaline  solution,  rinsed  out  and  washed  with  sulphuric 
acid — chromate  "cleaning  solution."  It  was  then  washed  with  pure 
water  and  finally  steamed  out. 

Preparation  of  Mercury. — The  mercury  used  as  enclosing  liquid  was 
washed  with  nitric  acid  as  described  by  Hildebrand26  and  finally  distilled 
under  a  partial  vacuum  with  a  slow  steam  of  air  bubbling  through  it. 

Loading  the  Tensimeter. — The  mercury  was  introduced  into  the  U 
and  after  bringing  to  a  vertical  position,  conductivity  water  was  distilled 
through  the  side  arm  (i)  into  the  bulb  (a).  After  about  2  cc.  had  been 
collected,  the  side  arm  was  sealed  oif  at  (h).  This  then  gave  water  es- 
sentially free  from  dissolved  air  except  for  the  air  in  the  tensimeter.  The 
hydrate  was  then  put  into  the  bulb  (d)  which  was  then  sealed  on  at  (/). 
Experience  showed  that  one  of  the  main  precautions  necessary  was  in- 
volved in  making  the  seals  in  the  glass.  In  an  extreme  case  which  was 
encountered,  there  was  a  slow  leak  that  during  the  course  of  three  weeks 
amounted  to  20  mm.  of  mercury  pressure.  An  oxygen  gas  torch  was 
therefore  used,  and  all  seals  were  gone  over  carefully. 

The  tensimeter  was  placed  in  a  horizontal  position  so  that  the  mercury 
was  in  bulb  c,  and  attached  at  e,  through  pressure  tubing,  to  a  "Cenoc" 
pump  capable  of  producing  a  1-mm.  vacuum.  The  pump  was  then  started 
and  a  vacuum  of  2  or  3  mm.  maintained  for  from  1  to  2  hours.  During 
this  period  the  tensimeter  was  alternately  heated  and  cooled.  When 
gently  warmed  the  mercury  boiled.  All  parts  of  the  instrument  except 
the  bulbs  containing  the  mercury  and  water  were  heated  very  hot  during 
evacuation.  The  instrument  was  gently  tapped  to  aid  in  the  removal  of 
any  gas  trapped  by  the  water,  mercury  or  hydrate.  During  the  evacuation 
0.3  to  0.5  cc.  of  water  distilled  off.  Since  0.1  cc.  of  water  vapor  at  760 
mm.  pressure  occupies  about  160  cc.,  the  tensimeter  and  surface  of  the  hy- 
drate was  therefore  washed  with  a  stream  of  150  to  250  liters  of  water  vapor 
at  2  to  3  mm.  pressure. 

While  the  vacuum  was  maintained  the  tensimeter  was  brought  to  a  ver- 
tical position,  and  after  gently  warming  the  hydrate  until  slight  evidence 
of  dehydration  took  place,  it  was  sealed  off  at  (g).  From  two  to  four 
2<>  J.  Am.  Chem.  Soc.,  31,  933  (1909). 


16 


tensimeters  were  loaded  with  the  same  hydrate.  These  were  then  mounted 
as  shown  in  Figs  Ig  or  2B,  so  that  they  could  be  put  into  the  thermostat 
or  removed  whenever  desired.  Some  of  these  tubes  were  used  for 
months. 

Whenever  measurements  were  to  be  made,  2  tensimeters  were  loaded 
with  the  same  hydrate.  One  was  then  heated  until  the  salt  was  partially 
dehydrated.  After  setting  the  thermostat,  both  tensimeters  were  in- 
troduced, one  registering  a  lower  pressure  and  one  a  higher  pressure  than 

TABUS! 


Date 

April 


From  lower  From  higher 
H2O  pressure  HaO  pressure 


CuSO4.  5H2O-3H2O  AT  35.17° 
Pressure  in  millimeters  (corr.) 

Date 
April 


From  lower 
H2O  pressure 


1 

Started 

Started 

9 

16.4 

3 

15.7 

17.8 

10 

16.4 

4 

15.9 

17.5 

12 

16.5 

5 

16.1 

17.3 

14 

16.5 

6 

16.1 

16.9 

.  . 

.  . 

From  higher 
HaO  pressure 

16.9 
16.5 
16.5 
16.6 


16.3 


16.8 


Mean  final  value,  16.5 


they  should  at  the  point  of  equilibrium.  This  resulted  in  a  slow  increase 
in  pressure  in  one  case  and  a  decrease  in  the  other;  thus  equilibrium  was 
approached  from  both  a  higher  and  a  lower  pressure  at  the  same  time. 

Data  on  the  determination  of  one  point  at  35.17°  for  CuSO4.5H2O- 
3H2O  are  given  in  Table  I  and  graphically  expressed  in  Fig.  3. 


Fig.  3. 

The  vapor  pressures  were  obtained  in  this  manner  at  several  temperatures 
for  the  hydrates  CuSO4.5H2O-3H2O,  CuSO4.3H2O-lH2O,  MgSO4.- 
7H2O-6H2O,  CoSO4.7H2O-6H2O,  CdSO4.8/3H2O-lH2O,  MnSO4.5H2O- 
1H2O  and  for  the  saturated  solutions  of  MgSO4 .  6H2O,  CoSO4.6H2O,  Mn- 
SO4.  H2O.  Table  II  gives  in  detail  the  actual  readings  and  all  'calculations 
in  obtaining  the  vapor  pressures  for  the  system  CuSO4.5H2O-3H2O, 


17 

and  Table  III  shows  the  values  for  all  other  systems  without  the  detailed 
readings  and  averaging  as  shown  in  Table  II.    The  data  of  Tables  II 

II 


THE  VAPOR  PRESSURE  OF  CuSO4.5H2O-3H2O 

Variation 

Variation 
of 
Temp,  thermostat  Tensimeter 
°C.        =t  °C.              No. 

Diff.  of 
Hg.  levels 
corrected 
Mm. 

in  vapor 
Vapor    press,  of 
press.          H2O 
H2O     Thermostat 
Mm.          =fc  Mm. 

Vapor    press, 
of  hydrate 
Obs.                      Mean 
Mm.                       Mm. 

25.00 

0.004 

5-4-6* 

15 

.87-15.87-16.03 

23.76 

0.00 

7. 

9-7.9-7.7 

7 

.8 

30.17 

0.01 

5-6* 

20 

.8-20. 

4 

32.15 

0.02 

11. 

4-11.8 

11 

.6 

35.13 

0.01 

5-6* 

26 

.1-26. 

0 

42.49 

0.02 

16. 

4-16.5 

16 

.5 

36.65 

0.04 

5-4* 

27 

.7-27. 

9 

46.18 

0.10 

18. 

5-18.5 

18 

.5 

40.12 

0.04 

5-4* 

32 

.6-32. 

5 

55.69 

0.12 

23. 

1-23.2 

23 

.2 

45.07 

0.03 

5-4* 

39 

.4-39. 

5 

72.16 

0.12 

32. 

8-32.7 

32 

.8 

50.16 

0.03 

5-4* 

47 

.9-48. 

2 

93.38 

0.15 

45. 

5-45.2 

45 

.4 

55.29 

0.05 

5-4* 

58 

.1-57. 

9 

119.8 

0.29 

61. 

7-61.9 

61 

.8 

60.18 

0.05 

5-4* 

67 

.2-66. 

7 

150.7 

0.35 

83. 

5-84.0 

83 

.8 

60.46 

0..01 

6-5* 

67 

.0-67. 

2 

152.7 

0.07 

85. 

7-85.5 

85 

.6 

65.16 

0.04 

5-4* 

76 

.3-76. 

4 

188.9 

0.34 

112. 

6-112.5 

112 

.6 

70.16 

0.02 

8* 

85 

.0 

235.4 

0.21 

150.4 

150 

.4 

69.78 

0.02 

6-5* 

84 

.4-83. 

9 

231.5 

0.21 

147. 

1-147.6 

147 

.4 

80.13 

0.03 

6-5* 

96 

.9-97. 

7 

357.4 

0.45 

260. 

5-259.7 

260 

.1 

90.04 

0.03 

8-9-8* 

94 

.3-94. 

1-93.9 

526.8 

0.60 

432. 

5-432.7-432.9 

432 

.7 

CONDENSED  TABLE  OF  VAPOR 

t 


TABLE  III 

PRESSURES  DETERMINED  AS  INDICATED  IN  TABLE  II  FOR 

/  .  * 


°c. 

P 

°C. 

P 

°C. 

P 

CuSO4. 

3H2O 

CdSO4 

.  8/3H20 

CoS04. 

7H2O 

25.00 

5.6 

25.00 

17.8 

25.00 

17.0 

35.13 

11.8 

24.99 

17.6 

32.50 

28.7 

45.17 

22.1 

30.17 

25.5 

36.65 

38.0 

50.23 

30.9 

35.17 

35.0 

40.18 

48.1 

65.11 

77.7 

40.12 

47.8 

40.22 

48.4 

80.08 

183.1 

40.25 

48.7 

45.07 

66.0 

80.05 

183.5 

45.07 

63.8 

45.17 

66.5 

MgSO4. 

7H2O 

45.17 

64.7 

50.16 

84.9 

25.00 

12.7 

50.16 

84.5 

55.29 

107.7 

32.40 

22.8 

50.23 

84.0 

60.22 

134.9 

36.65 

31.4 

55.29 

110.2 

65.16 

167.3 

36.65 

31.5 

60.22 

140.0 

70.16 

204.3 

40.12 

40.1 

65.16 

175.7 

MnSO4.  H2O 

40.19 

40.6 

70.16 

218.6 

25.00 

19.8 

40.22 

40'.6 

75.87 

279.3 

24.99 

20.1 

45.07 

57.2 

80.03 

334.3 

30.17 

27.1 

45.21 

58.0 

80.03 

334.6 

32.47 

31.1 

50.16 

78.6 

90.04 

500.3 

35.17 

37.0 

50.29 

79.0 

... 

.  .  . 

36.65 

40.4 

55.26 

99.3 

. 

•  .  . 

40.17 

49.1 

55.33 

99.6 

.  .  . 

.  .  . 

40.19 

49.3 

60.18 

123.5 

.  .  . 

.  .  . 

45.14 

64.4 

65.15 

153.0 

.  .  . 

60.29 

138.7 

65.11 

153.9 

65.16 

174.7 

69.74 

185.7 

... 

18 

and  III  are  expressed  graphically  in  Fig.  4.  Those  marked  with  an  aster- 
isk are  the  tensimeters  in  which  equilibrium  was  approached  from  the 
higher  vapor  pressure. 


To  read  Curves  ahng  Ordinate 
Ntn  I.  S.M  A  N,      Directly 
M>*.  V&  VI,  Subtract  SO 
A/03.W&VS.      " 
A/osJX&X.        •'         160 


Discussion  of  the  Accuracy  of  the  Results 

As  this  investigation  was  undertaken  in*  order  to  obtain  results  with 
the  greatest  degree  of  accuracy,  a  brief  review  of  the  precautions  taken  in 
the  experimental  work  and  an  estimation  of  the  accuracy  of  the  results 
obtained  should  be  made.  The  preparation  of  pure  hydrates  and  pure 
water  have  already  been  described.  The  necessity  of  knowing  the  exact 
temperatures  was  also  recognized  and  a  determination  of  them  accom- 
plished by  standardizing  the  thermometers  used,  as  described  before.  En- 
closed "permanent  gas,"  which  is  recognized  as  one  of  the  chief  difficulties, 


19 

was  removed  by  warming  and  washing  the  surface  of  all  materials  and  appa- 
ratus with  a  stream  of  water  vapor  at  low  pressures  as  described  previously. 
That  it  was  eliminated  is  evident  from  the  fact  that  instruments  set  up 
at  different  times  and  containing  different  amounts  of  water,  hydrate, 
mercury,  and  exposed  glass  surface,  gave  satisfactory  duplication  in  re- 
sults. A  further  evidence  is  the  fact  that  the  same  instrument  used  over 
a  long  period  of  time  and  at  many  different  temperatures  gave  dupli- 
cate results  when  brought  back  to  the  temperatures  used  in  the  earlier 
experiments.  In  fact,  new  instruments  when  set  up  duplicated  the  re- 
sults of  others  that  had  been  prepared  months  before.  A  further  possibility 
of  error  results  in  the  reading  of  the  mercury  columns  and  the  regulation 
of  the  thermostat.  The  reading  of  the  heights  of  the  mercury  columns 
could  readily  be  made  with  an  error  of  not  more  than  ±0.1  mm.  and  if  this 
were  -f-0.1  mm.  in  one  reading  and  — 0.1  mm.  in  the  other,  the  error 
would  be  cumulative,  or  a  total  error  of  ±  0.2  mm.  The  variation  in-  the 
temperature  also  leads  to  some  error,  as  a  change  of  0.69°  at  25°  causes 
a  change  of  1  mm.  in  the  vapor  pressure  of  water,  and  0.049°  causes  the 
same  change  at  90°.  At  the  lower  temperatures  the  regulation  was  better 
than  0.005°  and  at  the  higher  within  0.03°.  The  maximum  error  due  to 
variations  in  temperature  of  the  thermostat  would  therefore  be  less  than 
0.01  mm.  at  25°  and  about  0.6  mm.  at  90°. 

Whether  these  apparent  maximum  errors  are  real  is  a  pertinent  question. 
At  the  lower  temperatures,  where  equilibrium  is  reached  slowly,  an  error 
of  ±0.6  mm.  due  to  thermostat  regulation  would  become  very  serious, 
but  no  such  difficulty  exists.  On  the  other  hand,  at  the  higher  tempera- 
tures equilibrium  is  reached  quickly  and  as  the  temperature  of  the  thermo- 
stat is  taken  at  each  reading,  the  pressure  at  that  temperature  is  probably 
close  to  the  equilibrium  pressure.  Since  the  temperatures  and  pressures 
recorded  at  these  higher  points  are  averages  of  several  readings,  the  error 
is  far  below  the  apparent  error.  If,  however,  we  accept  the  individual 
maximum  apparent  errors  on  one  tube  as  ±0.2  mm.  at  the  lower  tempera- 
tures and  =±=0.6  mm.  at  the  higher,  the  shape  of  the  vapor-pressure  curve 
will  be  very  near  the  true  shape  and  any  result  obtained  by  reading  the 
curve  would  be  within  the  experimental  error.  An  error  of  ±0.2  mm. 
on  a  single  point  at  25°  on  a  value  of  10  mm.  represents  a  possible  error 
of  4%  while  an  error  at  90°  of  ±0.6  mm.  on  a  value  of  250  mm.  represents 
an  error  of  less  than  0.5%.  Accepting  the  maximum  apparent  errors, 
the  relative  error  at  the  higher  vapor  pressure  becomes  very  small;  but 
a  4%  error  at  the  lower  pressure,  if  real,  would  seriously  affect  the  abso- 
lute values.  It  must  be  borne  in  mind,  however,  that  every  tensimeter 
was  read  many  times,  and  the  average  reading  taken  as  the  final,  and  that 
no  point  was  determined  by  only  1  tube  but  by  the  average  of  at  least  2. 
In  order  to  make  this  4%  relative  error  real,  every  reading  would  need 


20 


to  have  a  maximum  error  and  always  in  the  same  direction,  that  is,  it 
would  always  be  necessary  to  read  1  tube  consistently  0.2  mm.  high  and  the 
other  consistently  0.2  mm.  low  in  order  that  each  individual  value  should 
have  a  maximum  error.  That  such  an  error  could  not  be  possible  is  better 
illustrated  by  considering  the  method  of  determining  the  point  of  equi- 
librium and  the  procedure  in  reading  the  value  already  shown  in  Table 
I  and  Fig.  2,  which  shows  that  the  maximum  ±  0.2  mm.  error  exists  for 
only  2  of  the  18  points.  It  is  evident,  therefore,  that  the  real  error  is  much 
below  the  maximum  possible  error  obtained  when  considering  the  extreme 
cases. 

Interpretation  of  the  Results 

The  most  evident  lessons  from  the  results  of  this  investigation  can  be 
learned  by  a  study  of  the  curves  in  Fig.  4.  As  shown  in  Fig.  4,  Curves 
V,  VII  and  IX,  a  transition  takes  place  and  in  two  cases  is  located  by 
the  intersection  of  two  curves. 

In  the  case  of  MgSO4.7H2O-6H2O  (see  Curve  V)  the  hydrate  is  stable 
up  to  48.4°.  The  upper  segment  is  the  vapor-pressure  curve  for  a  satu- 
rated solution  of  MgSO4. 6H2O. 
This  verifies  the  findings  of 
Van  der  Heide27  who  showed  by 
the  dilatometer  that  the  transi- 
tion point  is  between  48°  and 
48.5°.  The  solubility  data  as 
collected  by  Seidell28  gives  an 
indication  of  this  point  (see 
Fig.  5).  Too  much  cannot  be 
expected  from  these  solubility 
data,  as  the  14  points  are  taken 
from  12  observers.  The  posi- 
tion of  this  point  is  only  ap- 
proximately located  by  solubility  data. 

CoSO4. 7H2O-6H2O  is  stable  up  to  45.1  °  (see  Curve  VII).  No  previous 
record  of  this  point  has  been  found.  Marignac,29  however,  working  with 
the  dilatometer,  found  a  transition  point  at  40.8°  and  by  a  thermometric 
method  at  40.6°;  our  results  show  no  indication  of  a  transition  of  any 
kind  around  40°. 

The  vapor-pressure  curve  for  a  saturated  solution  of  MnSO4.H2O, 
and  one  point  at  25°  for  the  hydrate  are  given  (see  Curve  IX).  The 
hydrate  is  not  stable  above  27°.  The  transition  point,  therefore,  cannot 

27  Z.  physik.  Chem.,  12,  418  (1893). 

28  "Solubilities  of  Inorganic  and  Organic  Substances,"  2nd  ed. 

29  Assn.  Chem.  Phar.,  97,  247  (1856). 


21 

be  located  from  our  data  by  this  method.  The  transition  point  has  been 
found  to  be  27°  by  Cottrell.30 

The  vapor  pressure  of  CdSO4.8/3H2O-lH2O,  determined  over  a  wide 
range  and  plotted  in  Curve  III,  gives  little  evidence  of  any  transition 
point.  It  must  be  noted,  however,  that  the  curve  does  not  correspond 
with  that  of  the  water  or  with  the  other  curves  given.  This  might  not  be 
apparent  at  first,  as  it  is  "tangentially"  parallel  with  the  water  curve  but  not 
parallel  in  the  sense  that  a  second  curve  is  when  measured  by  the  distance 
along  the  ordinates.  The  peculiarity  in  vapor-pressure  change  seems  to 
be  paralleled  by  the  peculiar  change  in  solubility  *  Etard31  found  the  great- 
est solubility  to  be  at  68°  while  Mylias  and  Funk32  found  it  to  be  between 
73.5°  and  74.5°,  at  which  point  the  solubility  commenced  to  drop  very 
rapidly.  If  the  solubility  decreases  as  shown  in  the  curve  of  Fig.  5,  the 
solution  grows  rapidly  more  dilute  above  this  point  and  thus  results  in 
causing  the  vapor  pressure  to  increase  more  rapidly.  It  must  be  noted 
that  the  real  change  in  solubility  is  small. 

In  the  results  for  the  vapor  pressure  of.  CdSO4. 7H2O  given  above,  many 
points  were  duplicated  several  times  and  equilibrium  was  established  very 
carefully.  Although  the  shape  of  the  curve  gives  little  evidence  of  a  transi- 
tion point  at  74°,  it  must  be  noted  that  it  is  peculiarly  flat  in  this  region. 
It  will  be  shown  below,  however,  that  there  are  certain  theoretical  inter- 
pretations possible  of  its  behavior. 

Application  of  Results 

Heat  Reaction. — The  heat  reaction,  which  may  in  this  case  be  con- 
sidered as  the  latest  heat  of  vaporization,  has  been  calculated  for  each 
of  the  hydrates  investigated  except  for  that  of  manganese  sulfate 
which  was  omitted  for  lack  of  data.  In  the  reaction  AB.#H2O  + 
1H2O  ±^  AB.  (1  -f  #)H2O  -h  Qp,  Qp  is  given  by  the  following  relation: 

Qp  =  RT2      T  *    Qp  is  therefore  the  heat  of  reaction  per  mole  of  water 

at  constant  pressure;  p  is  the  pressure  of  water  vapor  of  the  system  at 
equilibrium  at  the  absolute  temperature  T.  Since  the  value  Q  is  very 
sensitive  to  changes  in  p  and  T,  the  extreme  values  may  easily  diverge 
=*=4%  from  the  mean  value  of  Q  for  the  system  being  considered.  Q  is 
determined  for  each  system  by  substituting  the  experimental  values 
recorded  in  Tables  II  and  III  in  the  formula,  and  are  given  in  Table  IV. 
The  average  value  for  CuSO4.3H2O  is  13,256;  for  CuSO4.5H2O, 
13,268;  for  CdSO4.8/3H2O,  11,170;  for  MgSO4.7H2O,  14,035;  for  Mg- 
SO4.6H2O,  saturated  solution,  9,741;  for  CoSO4.7H2O,  12,795;  and  for 
CoSO4.6H2O,  saturated  solution,  9,760. 

3°  J.  Phys.  Chem.,  4,  651  (1900). 

3i  Ann.  chim.  phys.,  [7]  2,  536  (1894). 

«  Ber.,  30,  824  (1897). 


22 


Values  were  also  taken  from  the  smooth  curye  III,  Fig.  4,  and  used  in 

making  similar  calculations  and  are  given  in  the  Appendix  to  Table  IV. 

It  may  be  observed  that  the  calculated  values  for  Q  for  the  systems 


TABLE  IV 


Temperature 
Interval 


Qp 
Calories 


CuS04.3H20-lH20 

25.00-35.13  13434 

35.13-45.17  12183 

45.17-50.23  13542 

50.23-65.11  13469 

65.11-80.07  13651 

Average  13256 

CuSO4.5H2O-3H2O 

25.00-30.17  13789 

30.17-35.13  13192 

35.13-40.12  13100 

40.12^5.17  13840 

45.07-50.16  13051 

50.16-55.29  12678 

55.29-60.46  13713 

60.46-65.16  12990 

65.16-69.78  13401 

69.78-80.13  13205 

80.13-90.04  12993 

Average  13268 


MgS04.7H20-6H20 


25.00-32.40 
32.40-36.65 
36.65-40.19 
36.65-40.22 
40.19^5.21 
40.22-45.21 
45.21-48.42* 

Average 

*  From  curve 


14308 
14299 
13821 
13706 
14076 
14163 
13871 
14035 


MgSO4.6H2O-Saturated  Solution 


48.42-50.29 
50.29-55.26 
50.29-55.33 
55.26-60.18 
55.33-60.18 
60.18-65.11 
65.11-69.74 

Average 


9943 
9708 
9703 
9633 
9644 
9996 
9563 
9741 


Temperature 

QP 

Interval 

Calories 

CdS04.  — 

H20-1H20 

3 

24.99-30.17 

12855 

30.17-35.17 

11763 

35.17-40.25 

12479 

35.17-40.12 

12078 

40.25-45.17 

11440 

40.12^5.07 

11521 

45.17-50.23 

10548 

45.07-50.23 

10895 

50.23-55.29 

11317 

55.29-60.22 

10557 

60.22-65.16 

10298 

65.16-70.16 

10077 

70.16-80.03 

10381 

80.03-90.04 

10247 

CoS04.7H20-6H20 


25.00-32.50 
32.50-36.65 
36.65-40.18 
36.65-40.22 
40.18-45.07 
40.22-45.07 

Average 


12638 
12719 
12873 
13066 
12811 
12664 
12795 


CoSO4.6H2O-Saturated  Solution 


45.17-50.16 
50.16-55.29 
55.29-60.22 
60.22-65.16 
65.16-70.16 


10005 
9779 
9932 
9761 
9454 


CuSO4. 5H2O-3H2O  and  CuSO4. 3H2O-1H2O  vary  on  both  sides  of  the  mean 
value  but  no  steady  increase  or  decrease  of  value  is  indicated.     It  is  note- 


23 

worthy  that  both  systems  show  almost  identical  mean  values  for  the  cal- 
culated heat  of  vaporization  over  the  same  temperature  range.  In  the 
case  of  MgSO4.7H2O-6H2O,  the  calculated  values  of  Q  for  the  hydrate 
and  the  saturated  solution  are  strikingly  different.  In  fact  this  change 
is  so  evident  that  the  approximate  position  of  the  transition  point  may  be 
closely  located.  In  order  to  appreciate  this  check  on  transition  point, 
it  must  be  borne  in  mind  that  the  values  used  in  the  calculations  were 
experimental. 

In  the  case  of  CoSO4.7H2O-6H2O,  the  same  striking  observations  may 
be  made  as  in  the  case  of  the  MgSO4 .  7H2O-6H2O  system. 

In  the  case  of  CdSO4.8/3H2O-lH2O,  the  calculated  value  of  Q  shows 
a  fairly  steady  decrease  up  to  about  70°,  above  which  its  value  remains 
nearly  constant.  This  is  shown  more  clearly  by  the  data  given  in  the 
Appendix  to  Table  IV  which  were  calculated  from  values  obtained  by 
reading  points  at  5°  intervals  on  the  smooth  curve  in  Fig.  4,  Curve  III. 

TABLE  IV  APPENDIX 

CdS04.  |-H2O-1H20 

o 

Temperature  Pressures  Qp 

Interval  from  curve  Calories 

25.00-30.00  17.6  25.2  12886 

30.00-35.00  25.2  34.8  11976 

35.00-40.00  34.8  47.5  11924 

40.00-45.00  47.5  63.6    '  11551 

45.00-50.00  63.6  83.3  11019 

50.00-55.00  83.3  108.7  11209 

55.00-60.00  108.7  139.6  10552 

60.00-65.00  138.6  174.2  10230 

65.00-70.00  174.2  217.3  10189 

70.00-75.87  217.3  279.2  10158 

75.87-80.00  279.2  333.8  10587 

80.00-90.04  333.8  500.3  10269 

Relation  between  Temperature  and  Pressure. — Very  few  attempts  to 
find  a  mathematical  expression  for  the  variation  of  pressure  with  tempera- 
ture in  the  case  of  hydrates  have  been  reported  in  the  literature.  This 
is  probably  due  to  the  short  temperature  ranges  covered  by  previous 
investigators  and,  therefore,  insufficient  data  on  the  subject.  Pareau,23 
however,  noted  that  his  curves  were,  in  general,  similar  to  that  of  pure 
water.  Baxter  and  Lansing24  plotted  the  logarithm  of  the  aqueous  pres- 
sure against  the  reciprocal  of  the  absolute  temperature  and  obtained  very 
nearly  straight  lines.  These  lines  are  represented  very  closely  by  an 

jD 

equation  developed  by  Antoine26  of  the  form  log  P  =  A  +  j   .   c  where 

23  Pareau,  Pogg.  Ann.,  1,  39  (1877). 

24  Baxter  and  Lansing,  THIS  JOURNAL,  42,  419  (1920). 
«  Antoine,  Compt.  rend.,  110,  632  (1890). 


24 


A,  B  and  C  are  constants.  The  values  for  log  p  and  l/T  have  been  cal- 
culated from  the  experimental  values  given  in  Tables  II  and  III,  and  were 
then  used  in  plotting  lines,  in  the  same  manner  as  was  done  by  Baxter 
and  Lansing24  for  their  vapor-pressure  data.  They  are  shown  in  Fig.  6. 
These  points,  both  for  the  hydrates  and  saturated  solutions,  fall  within 


CuSQ4-(5H,0-SH, 


To  READ  LIMES  ALOHC  ABSCISSA 

Nos.I,X,MANt>lV 
SUBTRACT  .250  FROM  Loe p 
Mas.  V,  VI  AND  Vff  D/HCCTLY  — 


Fig.  6. 

the  limits  of  experimental  error  on  straight  lines.    This  is  not  altogether 
unexpected  for  if  the  equation,  Q  =  RT2      *     is  integrated,  it  reduces 

1  r> 

to  the  following  form,  -  =  -  In  p  + 1,  where  I  is  the  integration  constant. 

If,  now,  Q  is  constant,  then  R/Q  is  a  constant  and,  therefore,  the  slope 
of  a  straight  line. 


25 


The  results  shown  on  these  lines  are  especially  striking.  The  transition 
points  are  very  definitely  located  by  the  intersection  of  the  straight  lines. 
They  intersect,  in  the  case  of  the  MgSQ4. 7H2O-6H2O,  Line  III,  at  a  point 
corresponding  to  48.4°  and  in  the  case  of  CoSO4 .  7H2O-6H2O,  Line  II,  at 
45.9°.  These  points  agree  very  closely  with  those  indicated  on  the  vapor 
pressure  curves  in  Fig.  4. 

The  case  of  CdSO4 . 8/3H2O-lH2O,  Line  V,  gives  a  very  definite  indication 
of  a  transition  point  at  41.5°.  This  seems  quite  likely  for  several  reasons. 
If  one  refers  to  the  vapor-pressure  Curve  III,  Fig.  4,  it  will  be  noted  that 
there  is  a  peculiar  flattening  in  its  shape  about  this  point.  A  reference 
to  the  solubility  diagram  Fig.  5,  shows  that  the  solubility  begins  to  change 
more  rapidly  around  this  point.  The  change  in  Q  is  as  great  from  25° 
to  45°  as  from  45°  to  90°.  It  is  also  a  well-known  fact  that  in  preparing 
cadmium  sulfate  for  the  standard  Clark  and  Weston  cell  the  crystals 
must  be  prepared  by  slow  evaporation  at  low  temperatures.  While  little 
is  known  of  the  nature  of  the  transition,  it  is  possible  that  it  is  a  molecular 
rearrangement  resulting  in  a  different  crystal  form. 

The  point  at  74.0°,  described  by  Mylias  and  Funk,22  is  not  shown  by 
any  indication  of  an  intersection  of  lines.  It  is  possible  that  at  the  higher 
pressures,  since  a  small  change  in  log  p  represents  a  considerable  change 
in  pressure,  the  irregularity  in  the  normal  shape  of  a  curve  below  and 
above  the  transition  point,  as  shown  around  74°  in  the  vapor-pressure 
curve  of  cadmium  sulfate,  may  not  be  given  definitely  by  logarithmic 
expressions.  In  such  a  case  the  two  straight  lines  above  and  below  the 
transition  point  may  be  so  nearly  similar  in  direction  that  the  possibility 
of  a  sharp  intersection  is  eliminated. 

Summary 

A  thermostat  has  been  assembled  for  very  accurate  control  of  tempera- 
tures between  25°  and  100°. 

The  static  method  has  been  employed  for  measuring  vapor  pressures 
of  hydrates,  and  a  Bremer-Frowein  tensimeter,  of  special  design,  and  ma- 
nipulation to  eliminate  the  usual  errors,  have  been  described. 

Equilibrium  has  been  reached  from  higher  and  lower  pressures  simul- 
taneously for  each  point  investigated. 

The  vapor  pressures  for  CuSO4.5H2O,  CuSO4.3H2O,  CdSO4.8/3H2O, 
CoSO4.7H2O,  MgSO4.7H2O,  and  MnSO4.5H2O  crystals,  and  for  the 
saturated  solutions  of  some  of  these  at  various  temperatures  between 
25°  and  90°,  have  been  determined. 

Certain  new  transition  points  have  been  located,  for  CoSO4.7H2O  at 
45. 1°  and  for  CdSO4.8/3H2O  at  41.5°. 

The  transition  point  for  MgSO4.7H2O-6H2O,  previously  found  by 
Van  der  Heide  to  lie  between  48.0°  and  48.5°,  has  been  located  at  48.4°. 


26 

It  has  been  shown  that  the  value  of  Q  usually  changes  most  abruptly 
at  the  transition  point  and  that  it  is  nearly  constant  so  long  as  the  same 
phases  are  present.  On  account  of  this  fact  most  transition  points  are 
readily  located  by  the  intersection  of  the  lines  drawn  through  the  points 
determined  by  the  log  p  and  l/T  relation. 


VITA 

Eric  Randolph  Jette  was  born  in  Lancaster,  Penna.,  September  30, 
1897.  His  primary  and  intermediate  education  was  received  in  the  pub- 
lic schools  of  that  city.  In  1914,  he  entered  Franklin  and  Marshall  Col- 
lege as  a  special  student  in  chemistry  and  later  changed  to  the  regular 
science  course.  He  received  the  degree  of  Bachelor  of  Science  in  June 
1918.  From  this  time  until  January  1919,  he  was  with  the  Chemical 
Warfare  Service,  stationed  at  the  American  University,  Washington,  D.  C. 
He  was  then  employed  by  Arthur  D.  Little,  Inc.,  Cambridge,  Mass., 
until  September  1919,  when  he  entered  Columbia  University  as  a  graduate 
student.  He  received  the  degree  of  Master  of  Arts  in  June  1920.  During 
the  last  three  years,  he  has  been  an  Assistant  in  the  Department  of  Chem- 
istry of  Columbia  University. 


Gaylord  Bros. 

Makers 
Syracuse,  N.  Y. 

«T.  JAN  21,  1908 


531  Si 


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